Hitherto, a method has been proposed to mix an electrically insulating thermoplastic resin with an electrically conductive substance for the purpose of forming a resin composition capable of being applied with an electrostatic coating, and such a method is partly practiced. As the electrically conductive substance generally added to the thermoplastic resin, there may be mentioned organic compounds such as ionic surfactants, nonionic surfactants and polymeric antistatic agents having a polyethylene glycol unit and an ionic functional group, as well as inorganic substances such as carbon black, carbon fibers, metal fibers, metal powder and metal oxides. In particular, resin compositions comprising conductive carbon black are most actively investigated in consideration of the balance of cost and physical properties.
Since addition of electrically conductive carbon black into a resin considerably reduces the fluidity and impact resistance of thereof, however, it is a common practice to use carbon black together with an additive such as an impact modifier. The use of an impact modifier, however, causes a reduction of rigidity and heat-resistant rigidity. Thus, there are occasionally cases where such a resin composition cannot be used as a material for producing automobile exterior parts.
When a reinforcing material such as an inorganic filler is additionally compounded in the resin composition in an attempt to improve rigidity and dimensional property thereof, the impact resistance is deteriorated by the addition of the inorganic filler. Moreover, any of the inorganic filler, impact modifier and electrically conductive carbon black mostly serves to reduce the fluidity of the resin. Therefore, it is a great problem how to balance these properties.
When thermoplastic resin materials are utilized for automobile exterior parts such as back doors, bumper fascias, fenders and door panels, a far higher level of performance is recently demanded of them as compared with those for conventional component parts. In this circumstance, materials conventionally used for automobile exterior parts cannot be used.
That is, a material for automobile exterior parts is required to exhibit the following characteristics at a higher level.
(1) Excellent fluidity: It is necessary for the resin material to have excellent fluidity in order to produce a large sized molded articles.
(2) Excellent impact strength: It is necessary for the resin material to have excellent high-speed surface impact strength in order to absorb impact energy by deformation.
(3) Excellent dimensional stability
(3-1) Low heat shrinkage: Automobile exterior parts are applied with a coating at a very high temperature. In high temperature conditions, crystallization of a crystalline resin proceeds to cause shrinkage. As a result, the dimension of the molded article changes before and after the coating. Thus, the article fixed at room temperature before coating considerably deforms after coating. In order to prevent the deformation, a measure is taken not to completely restrict the article but to use a slide jig to absorb the dimensional change. When the dimensional change is great, however, it is not possible to prevent the deformation. It is, therefore, necessary for the resin material to have low heat shrinkage.(3-2) Low linear expansion: A resin expands when exposed to a high temperatures at a time of coating. In this case, when a coefficient of linear thermal expansion is large, the resin is likely to deform by interference with its adjacent parts.
It is generally necessary to assume that automobiles are used under environment including a temperature in the range of about −20° C. to 80° C. Thus, it is necessary for the resin material to have as small a dimensional change (coefficient of linear expansion) as possible within the above temperature range.
(4) Excellent rigidity: It is necessary for the resin material to have appropriate rigidity at ambient temperature in order to be used as automobile exterior parts.
(5) Excellent heat-resistant rigidity: It is necessary for the resin material to have a high level of heat-resistant rigidity in order to avoid deformation even when exposed to high temperatures at a time of coating.
(6) Excellent conductivity: It is necessary for the resin material to have excellent electrical conductivity in order to be able to be coated by so called “electrostatic coating” technique in which a plate-like body (panel) of the electrically conductive resin material is electrically charged and is sprayed with a paint charged with opposite polarity. By utilization of mutually attracting action between the resin panel surface and the paint which are charged with opposing polarities, the adhesion efficiency of the paint can be improved.(7) Excellent coating adhesion: It is necessary for the resin material to provide sufficiently high adhesion to a paint so that the coated film is prevented from being peeled off.(8) Excellent appearance: Appearance needs to be good both before and after coating.(9) Excellent retention stability: Retention stability at a time of injection molding needs to be excellent.
With regard to resin compositions for producing automobile exterior parts, a polymer alloy of polyphenylene ether and polyamide has been most actively investigated. A great number of resin compositions and method of producing thereof have been thus far been proposed.
For example, proposals are made of a resin composition comprising polyphenylene ether (A), a polyamide (B) and a hydrogenated block copolymer elastomer (C) wherein a dispersion phase of (A) in the composition has a diameter of 0.6 μm or less; and a method of producing the impact resistant polyamide composition, characterized in that modified polyphenylene ether (a) obtained by reacting (A) with a 1,2-substituted olefin compound having a carboxylic acid group, a carboxylic anhydride group or an epoxy group in the presence of a radical generator, a modified hydrogenated block copolymer elastomer (c), obtained by reacting (C) with a 1,2-substituted olefin compound having a carboxylic acid group or a carboxylic anhydride group, and polyamide (B) are melted and kneaded together (Patent Document 1).
Also, there are proposals of an electrically conducting resin composition comprising polyphenylene ether, a polyamide and carbon black, wherein the composition is prepared by previously uniformly dispersing the carbon black in the polyamide, the resulting mixture being then mixed with the polyphenylene ether (Patent Document 2) and a resin composition prepared by first melting and kneading polyphenylene ether, an unsaturated polymer as an impact modifier, a functionalizing compound and optionally a part of a polyamide resin, the resulting mixture being then melted and kneaded with the remainder of the polyamide resin and electrically conductive carbon black having a low content of volatile matters (Patent Document 3).
A polyamide resin composition is further proposed which comprises a resin blend comprising a polyamide (A) having a specific ratio of a content of terminal amino groups to a content of terminal carboxyl groups, a hydrogenated block copolymer (B) obtained by partially or completely hydrogenating a copolymer of a vinyl aromatic compound and a conjugated diene compound, and a modified block copolymer (C) obtained by modifying the copolymer (B) with molecule units having a carboxyl group or group derived therefrom, the conjugated diene compound in the resin blend having an unsaturation degree of not exceeding 20%, components (A), (B) and (C) being contained in a specific ratio, and the polyamide resin composition further contains (D) an ethylene-α-olefin copolymer and (E) an olefin-based polymer having an acid anhydride group at its terminal end only (Patent Document 4).
The above conventional techniques, however, fail to achieve, simultaneously and in a well-balanced manner, the above-mentioned properties required as a material for automobile exterior parts.    Patent Document 1: Japanese Patent No. 2,557,637    Patent Document 2: Japanese Patent No. 2,756,548    Patent Document 3: Japanese Patent Application Laid-open (KOKAI) No. H10-310695    Patent Document 4: Japanese Patent No. 3,330,398
Patent Document 5 proposes a composition comprising a PA/PPE/SEBS alloy and various components compounded therein. Whilst the proposed composition succeeds to improve a balance of properties as compared with the hitherto proposed materials, the fluidity thereof is still in a low level because the composition contains PPE resin having poor melt fluidity. Therefore, there is a limit to the size of molded articles which can be produced.    Patent Document 5: Japanese Patent Application Laid-open (KOKAI) No. 2005-179546
Patent Document 6 proposes a resin composition comprising PA, an impact modifier and specific talc. The Document 6, however, does not specifically disclose methods or examples to improve coating adhesion and electrical conductivity which properties are required as a resin composition for producing automobile exterior parts. Since an improvement of electrical conductivity, namely an addition of carbon black, a carbon fiber or the like causes a deterioration of fluidity appearance and surface impact property, the electrical conductivity is one of the important points to attain balanced physical properties. Further, it is difficult per se to improve the coating adhesion. To improve the coating adhesion while satisfying the other properties in a balanced manner is a significantly difficult technical measure.
Because the Document 6 does not specifically disclose a method to improve the electrical conductivity, the disclosed composition cannot be used as a thermoplastic resin composition for producing automobile exterior parts.    Patent Document 6: Japanese Patent Application Laid-open (KOKAI) No. 2002-220531
As compositions similar to the resin composition of the present invention, the present inventors disclose, in Patent Document 7, a thermoplastic resin composition comprising PA, a vinyl aromatic block copolymer, a modified, hydrogenated block copolymer and an inorganic filler, and, in Patent Document 8, a thermoplastic resin composition comprising PA, an olefin-based polymer graft-modified with an unsaturated dicarboxylic acid, talc and pentaerythritol-based phosphite.
These compositions have highly balanced properties such as fluidity, impact resistance, rigidity, dimensional stability and appearance and can improve the electrical conductivity at the same time.
However, in Patent Document 7, a polyamide resin having a high molecular weight is used to compensate an insufficient improvement of the impact resistance attributed to a low degree of modification of the modified, hydrogenated block copolymer. Thus, it has been found that the balance between the fluidity and impact resistance is still unsatisfactory.
Additionally, it has been found that the combination disclosed in Patent Document 7 is insufficient to achieve the desired balance between fluidity, impact resistance, rigidity and dimensional stability.
In the case of Patent Document 8, the use of the phosphite compound can improve a balance between the impact resistance, rigidity and dimensional stability. However, because the entire amount of the olefin-based polymer, being an impact resistance improving component, is grafted with an unsaturated dicarboxylic acid, the fluidity is low. Thus, the balance of the properties of the proposed composition has been found to be still unsatisfactory.
Furthermore, in the case of the compositions of Patent Documents 7 and 8, when a melted resin is retained for a long period of time in a heated cylinder at the time of injection molding, appearance defects such as “silver streaks” and disadvantages such as deterioration of impact strength and rigidity have been found to be brought about.
Namely, with the prior art thermoplastic resin compositions, the balance of physical properties is insufficient for use as materials for producing automobile exterior parts. The application range of the prior art thermoplastic resin compositions is extremely limited under the present circumstances.    Patent Document 7: Japanese Patent Application Laid-open (KOKAI) No. 2004-331766    Patent Document 8: Japanese Patent Application Laid-open (KOKAI) No. 2004-346240